The detection of delimiter patterns contained in electromagnetic signals that are transmitted from RFID tags to RFID readers is a vital part in an RFID system, since most data exchange protocols used in RFID systems rely on the recognition of delimiter patterns. For instance, when applying a frame based data exchange protocol a data frame usually comprises a leading start of frame (SOF) pattern, followed by control flags and/or data bytes and a trailing end of frame (EOF) pattern signaling that the transmission of the data frame has been completed. The SOF and EOF patterns constitute delimiters with unique signal patterns. For illustration purposes only an example of a SOF pattern according to international standard ISO 15693 is shown in the diagram of FIG. 1. This SOF pattern (based on a carrier frequency fc: 13.56 MHz, load modulation, ASK with one subcarrier of fc/32 ˜423.75 kHz) comprises three parts, namely an unmodulated time period (56.64 μs), followed by 24 pulses of fc/32 (˜423.75 kHz), followed by a logic 1 which starts with an unmodulated time of 256/fc (˜18.88 μs), followed by 8 pulses of fc/32 (˜423.75 kHz). The overall length of the SOF pattern corresponds to the length of four data bits.
While, at first glance, it seems easy to detect this pattern by simple edge detection, in practice things are much more complicated. This will be appreciated when comparing the ideal SOF pattern of FIG. 1 with the signal stream according to the diagram of FIG. 2, which signal stream is an actual input signal received at an RFID reader from an RFID tag. As will be noted the signal stream of FIG. 2 is heavily distorted and superimposed by noise, so that the SOF pattern, which is contained in the signal stream between the boundaries of the light-gray shaded portion A, can hardly be recognized. It should be emphasized that the representation of the signal stream of FIG. 2 is not exaggerated, but such signals frequently appear in common environments. If SOF detection fails, the decoders within the RFID reader cannot compensate for that and consequently the data detection fails.
Document US 2001/0028691 A1 discloses a data carrier adapted to receive data in the form of data blocks, which data blocks include delimiter data and useful data. The data carrier includes delimiter data detection means adapted to detect delimiter data of a data block and to generate and supply at least one useful data start signal, in which also after the supply of the useful data start signal the delimiter data can be re-detected continually and the useful data start signal can be generated and supplied. The delimiter data detection means of this known data carrier works on the basis of bit level detection.
Unfortunately, the more immune the delimiter detection should be to distortions, the more computing power is needed (square performance in case of full correlation). By correlating the input signal with the whole delimiter pattern the best results can be achieved, but this approach is the worst case in respect of performance considerations.
An additional problem with delimiter detection in RFID systems is that the starting time of a response of an RFID tag to a request of an RFID reader cannot exactly be predicted, but tolerances in time have to be taken into account. This is illustrated in the diagram of FIG. 5. Here, an RFID reader upon sending a request REQ expects to receive a response of the RFID tag consisting of a leading SOF pattern, followed by the response data RESP, and a trailing EOF pattern. The SOF and EOF pattern act as delimiters. The SOF pattern must arrive at the RFID reader at an expected delimiter occurrence time t1 after the request REQ has been sent. However, the expected delimiter occurrence time t1 may jitter by the tolerance zone tz. For instance, the tolerance zone tz may sum up to 50% of the duration of a half-bit (Manchester coding). This tolerance zone tz cannot be used for data decoding, so that such RFID readers are very sensitive to distortions of the input signals, since even short distortions may result in detection errors. A solution for this problem is the above mentioned computational intensive correlation of the input signal with the delimiter pattern.
FIG. 6A shows the correlation result (4096 correlation values) of an ideal (sinusoidal) SOF pattern with the square SOF pattern of FIG. 1. The maximum of the correlation result has to be somewhere within the tolerance zone tz, which is represented by a gray shaded area. FIG. 6B shows a zoomed portion of the correlation function of FIG. 6A wherein the zoomed portion comprises the correlation values with the indices 1750 to 2350. According to standard ISO 15693 the tolerance zone tz spans about ±2.36 μs, i.e. about 4.7 μs, it includes 64 correlation values at fs=13.56 MHz. Correlation values are calculated each 74 ns which makes very high demands on the computational capacity of the RFID reader.